the effect of digital frame loss on audio teleprotection

8/3/2019 The Effect of Digital Frame Loss on Audio Teleprotection

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The

Effect of Digital

Frame Loss on

Audio Teleprotection

UTC REGION 2 MEETING


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Introduction

Audio Teleprotection was perfected overAudio Teleprotection was perfected overanalog communications systemsanalog communications systems

Digital is different!!!Digital is different!!!


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Overview of Digital Transmission

Serial versus parallel dataSerial versus parallel data

Framing patternsFraming patterns

Loss of frameLoss of frame


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1 24

1 8 WORD FRAMING BIT

T1 FRAME

ONE FRAME = 24 8-BIT WORDS (192 BITS)PLUS ONE FRAMING BIT (1 BIT)


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CALCULATING THE 1.544 Mbps T-1 RATE

STEP WHAT HAPPENS CALCULATION1. The 8-bit digital samples created

by PCM (voice channels only)

are grouped into 24 discrete time

slots created bt TDM. Each group

of time slots is called a T1 frame.

2. A framing bit is added to mark the

end of one frame and the beginning

of the next.

3. T1 frames are transmitted at the

rate of 8000 per second.

24 samples

8 bits per sample

192 information bits per frameX

192 information bits

1 framing bit

193 total bits per frame+

8000 samples

193 total bits1,544,000 bits per second (1.544Mbps)X


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The 1.544Mb/s is organized in an orderly, understandableThe 1.544Mb/s is organized in an orderly, understandableway. Framing provides this organization.way. Framing provides this organization.

A frame contains one sample(8-bits) from each of the DS-A frame contains one sample(8-bits) from each of the DS-1s 24 time slots. A framing bit is used to separate the1s 24 time slots. A framing bit is used to separate the

frames and indicate the order of information arriving at theframes and indicate the order of information arriving at the

receiving end.receiving end.

AA D1D1 frame contains 24 time slots, each carrying an eightframe contains 24 time slots, each carrying an eight

bit word and a framing bit. Bit eight of each word isbit word and a framing bit. Bit eight of each word isreserved for signaling information.reserved for signaling information.

D1 Framing


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THE D1 FRAMING PATTERN

1 24

1 8 WORD FRAMING BIT

24 8-BIT BYTES (192 BITS) X 8000 SAMPLES = 1.536 Mbps

SIGNALING BIT8-BIT BYTE


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D4 FRAMING/SUPERFRAME(SF)D4 FRAMING/SUPERFRAME(SF)

12 frames are grouped together to form a12 frames are grouped together to form a superframesuperframe..

Each frame contains 8-bits of each of the 24 time slotsEach frame contains 8-bits of each of the 24 time slots

and a framing bit(F-bit).and a framing bit(F-bit).

The twelve framing bits are combined into a 12-digitThe twelve framing bits are combined into a 12-digit

word that provides frame and signal management.word that provides frame and signal management.

To share signaling bits by all 12 frames in theTo share signaling bits by all 12 frames in the superframesuperframe,,

D4 framing uses a process called robbed bit signaling.D4 framing uses a process called robbed bit signaling.

The 8th bit of the DS-0s in the 6th and 12th frames areThe 8th bit of the DS-0s in the 6th and 12th frames are

reserved for signaling information.reserved for signaling information.

In summary, D4 framing improved signal quality byIn summary, D4 framing improved signal quality by

freeing more bits for customer information.freeing more bits for customer information.


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D4 SUPERFRAME (SF)

D4 Framing 1 0 0 0 1 1 0 1 1 1 0 0

Frame # F1 F2 F3 F4 F5 F6 F7 F8 F9 F10 F11 F12

1 Superframe = 12 Frames

1.5 ms = 2316 Bits

F 1 2 3 4 5 6 7 8

Time

Slot #

Bit#

1 2 24

1 Frame

125 Sec., 193 Bits

1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8


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EXTENDED SUPERFRAME (ESF)EXTENDED SUPERFRAME (ESF)

ESF expands theESF expands the superframesuperframe from 12 to 24 193-bitfrom 12 to 24 193-bit

frames. ESF was developed in order to be able toframes. ESF was developed in order to be able toevaluate system performance without disrupting serviceevaluate system performance without disrupting service

by testing the t-1 link.by testing the t-1 link.

Three fourths of the 24 control bits are reserved forThree fourths of the 24 control bits are reserved forevaluation of circuit performance.evaluation of circuit performance.

6 control bits are reserved for cycle redundancy6 control bits are reserved for cycle redundancy

check (CRC), a method of detecting errors duringcheck (CRC), a method of detecting errors duringtransmittal (2kb/s);transmittal (2kb/s);

12 control bits are reserved as a data link for12 control bits are reserved as a data link for

communication between transmitting and receivingcommunication between transmitting and receivingequipment at either side(4kb/s).equipment at either side(4kb/s).

6 control bits are use to managed signaling and framing.6 control bits are use to managed signaling and framing.


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EXTENDED SUPERFRAME (ESF)

1 24

1 24

1 8

DS1 FRAME

193 BITS (125sec)

DS0 CHANNEL

8 BITS (5.18 sec)

8th BIT IN FRAME # 6 A

12 B

18 C (A) DUPLICATE

24 D (B) DUPLICATE

SIGNALING INFORMATION

EXTENDED SUPERFRAME

4632 BITS (3 msec)

FRAMING BIT SUMMARY

F1

D C1 D 0 D C2 D 0 D C3 D 1 D C4 D 0 D C5 D 1 D C6 D 1

F24

D - FORCED TO 1 BY USER, TBD BY ATT, 4 KBPS EMBEDED OPERATIONS CHANNELC1 - C6 = CRC6 POLYNOMIAL FOR ERROR DETECTION, 2KBPS, 98.4% OF SINGLE AND

MULTIPLE ERRORS FOUND.

FRAMING PATTTERN = 2 KBPS.


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Serial Versus Parallel

Parallel transmission gives one characterat a time with hardware boundaries

Serial transmission has no hardboundaries

10101010101011100011101101111010101010101110001110110111

{Channel 1


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Framing Patterns

In order to find the boundaries a framingpattern is inserted

1PPP..PPP0PPP..PPP1PPP..PPP0PPP..PPP1

Framing Bits

Payload Data Bits

T1 has 192 (24x8) Payload Bits and 1 Frame Bit = 193 Bits


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Framing Patterns

However, bits are bits.Which ones are the frame bits?

110110011100011011010111000010110111011011101101

It takes time for a receiver to find the framingpattern and it is easily lost when bits are corrupted


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Loss of Frame

When a multiplexer is unframed the start

of frame pointer is pointing to the

wrong bit, shifting the channel pointersto the wrong bits


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Description of Problem

What causes loss of frameWhat causes loss of frame Media considerationsMedia considerations

Channel hoppingChannel hopping Pink noisePink noise

TeleprotectionTeleprotection


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What Causes Loss of Frame

Corrupted framing bits

Change in propagation time

Any disruption to the media of equipment can cause

A loss of frame.


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Media ConsiderationsFiber breaks, equipment failuresFiber breaks, equipment failures

When it goes it usually fails completelyWhen it goes it usually fails completely

or switches to a good pathor switches to a good path

Digital Microwave path fades,Digital Microwave path fades,

multipathmultipath

Comes and goes continuouslyComes and goes continuously


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Channel Hopping

Caused by frame misalignment

The frame misalignment is exactly amultiple of channel time or words.

(5.18 uS for 1 word in typical TDMmuxes)

The channel faithfully outputs dataintended for another channel or time

slot


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Pink Noise

Caused by frame misalignment. The alignment is off by less than one

channel or one word (5.18 us)Output circuits reproduce a complex

noise pattern including variousfrequency components


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Teleprotection Channel Hop

Designed to wait for trip frequenciesafter losing guard frequencies

A channel hop to anotherteleprotection channel with correct trip

frequencies for that channel


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Teleprotection Pink Noise

Designed to pull trip and guardfrequencies out of noisy channels

Pink Noise can mimic a noisy tripcondition

AGC circuits help bring the noise tothe proper level and a trip results


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Possible Solutions

PreventionPrevention DetectionDetection

AddressingAddressing Frequency DiversityFrequency Diversity

SquelchingSquelching


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Prevention

Build a system which hasBuild a system which hasRobust framing during corruptionRobust framing during corruption

Rapid detection of Loss Of FrameRapid detection of Loss Of FrameTakes action before a trip can occurTakes action before a trip can occur


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Detection

Detection time is limited by theframing pattern

At least two frames are required for

minimum detection time

Longer the frame slower the

detectionDont want to be too sensitive


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Channel Addressing

Each protection pair has a uniqueTX/RX address transmitted with data

Usually limited to digital teleprotection

Can be done with audio but not quicklyenough


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Frequency Diversity

Each protection pair has uniquefrequencies

Can be a maintenance problemDoes not protect against pink noise


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SquelchingThe Best Protection!The Best Protection!

Quickly detect loss of frame and

squelch output

A relaying grade multiplexer should do

this in less than 2 ms.

Not available with channel banks


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Test Results

Pink Noise No SquelchPink Noise No Squelch Pink Noise With SquelchPink Noise With Squelch

Channel Hopping No SquelchChannel Hopping No Squelch Channel Hopping With SquelchChannel Hopping With Squelch


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Pink Noise No SquelchValid Audio Tone Pink Noise

Loss of Frame


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Pink Noise - with Squelch

Loss of Frame

Valid Audio Tone Squelched Output

Channel Hopping No Squelch


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Channel Hopping No Squelch

Major utility sanctioned test Telecom grade T1 Mux 24 PCM voice

cards 23 channels at trip frequencies, 1 channe

at guard with RFL 9745 teleprotectionchannel receiver

Repetitive T1 breaksOne false trip output about every 20

minutes


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Channel Hopping With

Squelch

Major utility sanctioned test

RFL IMUX 2000 T1 Multiplexer

with DS0 squelching enabled Same setup and conditions as previous

test 17 Hours No False Trip Outputs

the effect of digital frame loss on audio teleprotection

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